The present invention relates to the storage and extraction of thermal energy on the basis of the latent heat of fusion of a storage medium.
Heat of fusion has been used successfully for buffering thermal energy as between a prime source of heat and a user, operating on an asynchronous basis in the sense that periods of development of heat do not or at least not completely coincide with periods of desired and required use. A typical example here is solar energy, the sun being the prime source and the user being, e.g. an electrical power plant. Obviously night time energy demands can be satisfied only through a buffer which must be filled in the daytime. Correspondingly, not all of the solar energy being collected could or should be used right away.
The thermal buffering as described requires employment of a heat storage medium. For example, salt mixtures are currently used to store thermal energy in the form of the latent heat of fusion of the mixture; that is to say large quantities of such a mixture are gradually melted by a heating process deriving its energy e.g. from a solar radiation concentrating system. If properly insulated, the molten salt retains the stored energy until a colder fluid is brought into heat exchange relation with the molten salt which will gradually solidify while heating the colder heat exchange fluid. Salts are used with advantage here, because the phase change temperature (i.e. the melting point) can be chosen and adapted to a high value so that one may generate steam, possibly even superheated steam at a sufficiently high temperature, which, however, is limited by the phase change temperature. Moreover, the latent heat of fusion should be rather high to obtain a rather high storage capacity on a caloric or watt-hour per unit volume basis.
The known heat exchange devices for such a system employ, for example, a storage tank and separate coils for the heat exchange fluids to be used for supplying heat to the tank and for extracting heat therefrom. It was found that formation of solid salts at and around the heat extraction coils significantly impedes the heat transfer from the still liquid state, particularly after a rather thick build-up in solid salt deposits on the heat extractor coils. One could (and, actually must) remove the salt deposits by, for example, a scraping mechanism, or the like. Without more or less continuous removal of the salt deposits heat transfer conditions change drastically throughout the heat extraction process amounting in effect in a significant drop in temperature of the heat extracting fluid. Such salt removal procedure is cumbersome and complicates the construction. Moreover, it seems rather undesirable to operate the heat buffer, being basically a passive device, under utilization of moving parts.